SWR Bass 750 (original, not 750x)

[chrisdski]

Looking for preamp pcb board# 170034B

Looking for power amp board pcb# 170033

For original Bass 750 bass amp head (not the 750x)

[nickb]

Looking for preamp pcb board# 170034B

Looking for power amp board pcb# 170033

For original Bass 750 bass amp head (not the 750x)

>>This<< might be what you need.

[chrisdski]

Yup, I've read that this may be a direct match to my power amp board. I'll have to trace it and see. Thanks! What I'm really needing and can't seems to find is the pre amp board schematic.

[g1]

What is the preamp board number?

[chrisdski]

What is the preamp board number?

Looking for preamp pcb board# 170034B

Looking for power amp board pcb# 170033

[musictronics]

Service Notes Attached...

Is this any help?

[chrisdski]

yes, a lot- thank you. Craigslist find for no money for a hobby project- knew the fuse blew so knew there is a problem. Power transformer specs out OK. I think the preamp board is fine though I did replace caps c1-c4 as they were starting to bulge. On the power board, all the power transistors Q9-Q16 meter open, so I think they are all blown. I have a 22R 2 watt resistor shot, trying to find it on the schematic to trace down whats around it. I'm using the blue board schematic for the power board.

[Enzo]

Um... output transistors fail often enough, but VERY rarely do they fail open. They almost always fail shorted. But even if one were to fail open,, to have ALL of them fail open is unheard of. Pull them and test them more thoroughly.

[chrisdski]

you are correct. Forgive my terminology ignorance. The output transistors are shorted- they show no resistance testing the 3 pins. The transistors will all be pulled.

[Enzo]

OK, better.

Your one resistor was burning up, but if all are shorted, then check ALL the resistors, they can burn open.

before condemning all the transistors, recheck them when they are out. they are all wired in parallel on the board, so one shorted one makes the rest appear shorted. In fact, pull Q14 first and then see if the remaining ones no longer seem to be shorted. Emitter to collector shorts often if not mostly short just one transistor on each polarity. Base to collector shorts usually take out the whole row.

[chrisdski]

I'll post a few pics of the boards before I start pulling it apart. Power amp board

[chrisdski]

Problems!

[chrisdski]

Pre amp board. Should be ok here. I replaced the 4 large caps as they had bulging tops. Will need help when I get to biasing the tube.

[Enzo]

Oh, I can help you right now "biasing the tube". Plug your tube into the socket. There, all biased.

Bias is adjusted for the power tubes in the output section of a tube amplifier. Little preamp tubes do not get adjustable bias, they are biased simply by the action of the circuit.

Your power amp also has a bias control in the middle, to set idle current through the transistor output and eliminate crossover distortion.

[g1]

In your photos there is evidence of previous butchery, so being a craigslist dead amp find, you will have to go real slow. There is the chance of incorrect parts, or incorrect installation of parts, such as orientation of small transistors, etc.
There is a screw jammed in off kilter, a plastic cap that has been melted by a solder iron, and that transistor in the middle looks like it has had it's legs clipped and then soldered back together again.